ExploreDNS/internal/traverse/traverser.go

package traverse

import (
	"context"
	"fmt"
	"net"
	"sync"
	"time"

	"github.com/hits/ExploreDNS/internal/dns"
	miekgdns "github.com/miekg/dns"
)

// TraverserConfig controls the behaviour of a Traverser.
type TraverserConfig struct {
	// MaxDepth caps the traversal depth.  Referrals at or beyond this depth
	// are rejected and reported as errors.
	MaxDepth int
	// QueryType is the DNS record type requested at each step (e.g. dns.TypeA).
	QueryType uint16
	// RootConfig controls how root servers are discovered at startup.
	RootConfig *dns.RootDiscoveryConfig
	// QueryConfig controls UDP/TCP transport settings for each DNS query.
	QueryConfig *dns.QueryConfig
	// RootAddrs may be supplied directly to skip root discovery.
	RootAddrs []net.IP
	// Hooks receives events during traversal (progress, resolve, result).
	Hooks *TraverserHooks
	// Fast controls cache sharing across branches. When true (default), child
	// branches inherit glue discovered by earlier branches via the shared root
	// cache, trading accuracy for speed. When false, each branch gets a
	// completely independent cache — slower but results are not contaminated by
	// sibling branch observations.
	Fast bool
}

// DefaultTraverserConfig returns a TraverserConfig with sensible defaults:
// max depth 20, query type A, fast mode on.
func DefaultTraverserConfig() *TraverserConfig {
	return &TraverserConfig{
		MaxDepth:    DefaultMaxDepth,
		QueryType:   dns.TypeA,
		RootConfig:  nil,
		QueryConfig: nil,
		RootAddrs:   nil,
		Fast:        true,
	}
}

// TraversalResult pairs a Referral (the query that was attempted) with the
// Response (the outcome of that query).  Response may be nil for referrals
// that were never processed (e.g. depth-limit rejections).
type TraversalResult struct {
	Referral *Referral
	Response *Response
}

// Traverser performs iterative DNS traversal from the root down to the target
// domain, following referrals and CNAME chains.
// Create one with NewTraverser; call Traverse to run a traversal.
type Traverser struct {
	config   *TraverserConfig
	exchange dns.ExchangeFunc
	visited  map[string]bool
	depth    int
	mu       sync.Mutex
}

// NewTraverser creates a Traverser using cfg.
// When cfg is nil, DefaultTraverserConfig is used.
func NewTraverser(cfg *TraverserConfig) *Traverser {
	if cfg == nil {
		cfg = DefaultTraverserConfig()
	}
	return &Traverser{
		config:   cfg,
		exchange: nil,
		visited:  make(map[string]bool),
		depth:    0,
	}
}

// SetExchange injects a custom exchange function, primarily for testing.
func (t *Traverser) SetExchange(fn dns.ExchangeFunc) {
	t.exchange = fn
}

// SetHooks attaches traversal event hooks to the Traverser.
func (t *Traverser) SetHooks(hooks *TraverserHooks) {
	if t.config == nil {
		t.config = DefaultTraverserConfig()
	}
	t.config.Hooks = hooks
}

// Traverse performs an iterative DNS traversal for name, starting from the root
// servers.  It returns all TraversalResults, including intermediate referrals
// and terminal outcomes.  Hooks are called for each event during the traversal.
// The context can be used to cancel a long-running traversal.
func (t *Traverser) Traverse(ctx context.Context, name string) ([]TraversalResult, error) {
	name = miekgdns.Fqdn(name)

	roots, err := t.discoverRoots(ctx)
	if err != nil {
		return nil, fmt.Errorf("root discovery: %w", err)
	}

	initial := NewReferral(name, t.config.QueryType, ".", 0, 1.0, nil)
	initial.Addresses = roots

	stack := NewStack(t.config.MaxDepth)
	stack.Push(initial)

	rootCache := NewInfoCache(nil)
	var (
		mu      sync.Mutex
		results []TraversalResult
	)

	for {
		select {
		case <-ctx.Done():
			return results, fmt.Errorf("traversal cancelled: %w", ctx.Err())
		default:
		}

		ref := stack.Pop()
		if ref == nil {
			break
		}

		var cache *InfoCache
		if t.config.Fast {
			// Fast mode: inherit glue from the shared root cache so earlier
			// branch discoveries are visible to later branches.
			cache = rootCache
			if ref.Parent != nil {
				cache = rootCache.Child()
			}
		} else {
			// Non-fast mode: every referral gets its own independent cache so
			// no cross-branch glue is reused, ensuring each path is resolved
			// from scratch.
			cache = NewInfoCache(nil)
		}

		if t.config.Hooks != nil {
			t.config.Hooks.emit(EventStart, TraversalResult{Referral: ref}, false)
		}

		resp := t.processReferral(ctx, ref, cache)

		result := TraversalResult{Referral: ref, Response: resp}
		if t.config.Hooks != nil {
			t.config.Hooks.emit(EventComplete, result, false)
		}

		mu.Lock()
		results = append(results, result)
		mu.Unlock()

		if resp.IsTerminal() {
			continue
		}

		if resp.Type == RespReferral {
			children := resp.ChildReferrals()
			for _, child := range children {
				if !stack.Push(child) {
					mu.Lock()
					results = append(results, TraversalResult{
						Referral: child,
						Response: &Response{
							Referral: child,
							Type:     RespError,
						},
					})
					mu.Unlock()
				}
			}
		}

		if resp.Type == RespCNAMEFollow {
			follow := resp.CNAMEFollowReferral()
			if follow != nil {
				// Detect CNAME loop: target name already appears in the ancestor chain.
				if follow.Parent != nil && follow.Parent.IsNameInChain(follow.Name) {
					mu.Lock()
					results = append(results, TraversalResult{
						Referral: follow,
						Response: &Response{
							Referral:     follow,
							Type:         RespCNAMELoop,
							ErrorMessage: fmt.Sprintf("CNAME loop detected: %s already in traversal chain", follow.Name),
						},
					})
					mu.Unlock()
				} else if !stack.Push(follow) {
					mu.Lock()
					results = append(results, TraversalResult{
						Referral: follow,
						Response: &Response{
							Referral: follow,
							Type:     RespError,
						},
					})
					mu.Unlock()
				}
			}
		}
	}

	return results, nil
}

func (t *Traverser) discoverRoots(ctx context.Context) ([]net.IP, error) {
	if len(t.config.RootAddrs) > 0 {
		return t.config.RootAddrs, nil
	}

	servers, err := dns.DiscoverRoots(ctx, t.config.RootConfig)
	if err != nil {
		return nil, err
	}

	var addrs []net.IP
	for _, srv := range servers {
		addrs = append(addrs, srv.AllIPs(false)...)
	}
	return addrs, nil
}

func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *InfoCache) *Response {
	if !ref.HasAddresses() {
		t.mu.Lock()
		visitedCopy := make(map[string]bool)
		for k, v := range t.visited {
			visitedCopy[k] = v
		}
		t.mu.Unlock()

		ref.Addresses = t.resolveGlueViaSystem(ctx, ref.Name, cache)
		if len(ref.Addresses) > 0 {
			ref.State = StateResolved
		} else {
			addrs, err := t.ResolveNS(ctx, ref.Name, cache, visitedCopy, t.depth)
			if err != nil {
				return &Response{
					Referral: ref,
					Type:     RespError,
				}
			}
			if len(addrs) > 0 {
				ref.Addresses = addrs
				ref.State = StateResolved
			} else {
				return &Response{
					Referral: ref,
					Type:     RespError,
				}
			}
		}
	}

	for _, addr := range ref.Addresses {
		resp := t.queryServer(ctx, ref, addr, cache)
		if resp != nil && resp.Type != RespSERVFAIL {
			return resp
		}
	}

	return &Response{
		Referral: ref,
		Type:     RespSERVFAIL,
	}
}

// ResolveNS resolves a nameserver hostname to its IP addresses by performing
// a fresh iterative traversal for that name, using cache to avoid repeated
// queries and visited to detect circular referrals.
func (t *Traverser) ResolveNS(ctx context.Context, nsName string, cache *InfoCache, visited map[string]bool, depth int) ([]net.IP, error) {
	if cache != nil {
		if addrs := cache.LookupGlue(nsName); len(addrs) > 0 {
			return addrs, nil
		}
	}

	if visited != nil {
		if visited[nsName] {
			return nil, &CircularReferralError{
				Name:  nsName,
				Chain: getVisitedNames(visited),
			}
		}
		visited[nsName] = true
	}

	if depth > DefaultMaxDepth {
		return nil, &UnresolvableNameserverError{
			Name:   nsName,
			Reason: "max depth exceeded",
		}
	}

	roots, err := t.discoverRoots(ctx)
	if err != nil {
		return nil, fmt.Errorf("root discovery: %w", err)
	}

	ref := NewReferral(nsName, dns.TypeA, ".", 0, 1.0, nil)
	ref.Addresses = roots
	ref.State = StateResolved

	traversalCache := NewInfoCache(nil)
	if visited != nil {
		for name := range visited {
			traversalCache.StoreGlue(name, []net.IP{})
		}
	}

	var addrs []net.IP
	var lastErr error

	stack := NewStack(DefaultMaxDepth)
	stack.Push(ref)

	for {
		select {
		case <-ctx.Done():
			return nil, fmt.Errorf("resolution cancelled: %w", ctx.Err())
		default:
		}

		current := stack.Pop()
		if current == nil {
			break
		}

		cacheForStep := traversalCache
		if current.Parent != nil {
			cacheForStep = traversalCache.Child()
		}

		if t.config.Hooks != nil {
			t.config.Hooks.emit(EventStart, TraversalResult{Referral: current}, true)
		}

		resp := t.processReferral(ctx, current, cacheForStep)

		if t.config.Hooks != nil {
			t.config.Hooks.emit(EventComplete, TraversalResult{Referral: current, Response: resp}, true)
		}

		if resp.Type == RespAnswer && len(resp.Decoded.Answers) > 0 {
			for _, rr := range resp.Decoded.Answers {
				if a, ok := rr.(*miekgdns.A); ok {
					addrs = append(addrs, a.A)
				}
				if aaaa, ok := rr.(*miekgdns.AAAA); ok {
					addrs = append(addrs, aaaa.AAAA)
				}
			}
			if len(addrs) > 0 {
				if cache != nil {
					cache.StoreGlue(nsName, addrs)
				}
				return addrs, nil
			}
		}

		if resp.Type == RespNXDOMAIN {
			lastErr = &UnresolvableNameserverError{
				Name:   nsName,
				Reason: "NXDOMAIN",
			}
			break
		}

		if resp.Type == RespSERVFAIL || resp.Type == RespError {
			lastErr = fmt.Errorf("server error resolving %s: %s", nsName, resp.Type)
			continue
		}

		if resp.Type == RespReferral {
			children := resp.ChildReferrals()
			for _, child := range children {
				if visited != nil && visited[child.Name] {
					continue
				}
				if !stack.Push(child) {
					lastErr = &UnresolvableNameserverError{
						Name:   nsName,
						Reason: "max depth exceeded during resolution",
					}
				}
			}
		}
	}

	if len(addrs) > 0 {
		return addrs, nil
	}

	if lastErr != nil {
		return nil, lastErr
	}

	return nil, &UnresolvableNameserverError{
		Name:   nsName,
		Reason: "resolution exhausted without answer",
	}
}

func (t *Traverser) queryServer(ctx context.Context, ref *Referral, server net.IP, cache *InfoCache) *Response {
	var msg *miekgdns.Msg
	var err error

	if t.exchange != nil {
		msg, err = t.iterativeQueryWithExchange(ctx, server, ref.Name, ref.Qtype)
	} else {
		msg, err = dns.Query(ctx, server, ref.Name, ref.Qtype, t.config.QueryConfig)
		if err == nil {
			msg = t.ensureRDFalse(msg, server, ref.Name, ref.Qtype, t.config.QueryConfig)
		}
	}

	if err != nil {
		return &Response{
			Referral: ref,
			Server:   server,
			Type:     RespError,
		}
	}

	resp := NewResponse(ref, server, cache)
	resp.Process(msg)
	return resp
}

func (t *Traverser) iterativeQueryWithExchange(ctx context.Context, server net.IP, name string, qtype uint16) (*miekgdns.Msg, error) {
	if t.config.QueryConfig == nil {
		return dns.IterativeQueryWithExchange(ctx, server, name, qtype, nil, t.exchange)
	}
	return dns.IterativeQueryWithExchange(ctx, server, name, qtype, t.config.QueryConfig, t.exchange)
}

func (t *Traverser) ensureRDFalse(msg *miekgdns.Msg, server net.IP, name string, qtype uint16, cfg *dns.QueryConfig) *miekgdns.Msg {
	if msg != nil && msg.RecursionDesired {
		if t.exchange != nil {
			ctx := context.Background()
			var err error
			msg, err = t.iterativeQueryWithExchange(ctx, server, name, qtype)
			if err != nil {
				return nil
			}
			return msg
		}
		msg.RecursionDesired = false
	}
	return msg
}

func (t *Traverser) resolveGlueViaSystem(ctx context.Context, name string, cache *InfoCache) []net.IP {
	if cache != nil {
		if addrs := cache.LookupGlue(name); len(addrs) > 0 {
			return addrs
		}
	}

	c := &miekgdns.Client{
		Net:          "udp",
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 5 * time.Second,
	}
	if deadline, ok := ctx.Deadline(); ok {
		remaining := time.Until(deadline)
		if remaining <= 0 {
			return nil
		}
		c.ReadTimeout = remaining
		c.WriteTimeout = remaining
	}

	fqdn := miekgdns.Fqdn(name)

	aMsg, _, err := c.ExchangeContext(ctx, newAQuery(fqdn), "127.0.0.1:53")
	if err == nil {
		var addrs []net.IP
		for _, rr := range aMsg.Answer {
			if a, ok := rr.(*miekgdns.A); ok {
				addrs = append(addrs, a.A)
			}
		}
		if len(addrs) > 0 {
			if cache != nil {
				cache.StoreGlue(name, addrs)
			}
			return addrs
		}
	}

	return nil
}

func newAQuery(name string) *miekgdns.Msg {
	m := new(miekgdns.Msg)
	m.SetQuestion(name, miekgdns.TypeA)
	m.RecursionDesired = true
	return m
}